The present invention relates to a process and apparatus for forming thin films on a substrate, for example, thin films of electrical conductors, semiconductors and/or insulating material, and particularly to forming large area thin films for photovoltaic solar cells.
Thin films constituting parts of solar cells may be deposited on a substrate (or on one or more layers overlying the substrate) using a number of processes including vapor deposition, electrodeposition, sputtering and various spray coating processes. The quality and characteristics of such films vary with the size of the thin film, the particular process used and with specific techniques utilized in carrying out a specific process. In a spray pyrolysis process, for example, the substrate is heated and a spray or atomized mist which includes one or more reactants is conducted by a gaseous carrier medium into contact with the heated substrate. The film may be formed from a single reactant which either releases or is the film substance or from two or more reactants which chemically react to release or form the film substrate. In either case, a chemical reaction takes place at the surface of the substrate causing the substance to be formed or deposited on the substrate surface as a thin film, while spent and unreacted reactants are volatized, evaporated or otherwise conducted away from the substrate with the gaseous carrier medium.
One goal in the production of thin film solar cells has been to economically produce large area (e.g., approximately 6 in. by 6 in. and larger) solar cells of high efficiency. Such large area solar cells may be used as large area solar cell panels in large solar energy conversion arrays or, when subdivided into smaller isolated cells, as solar batteries for devices such as watches and calculators. Heretofore, typical efficiencies obtained for large area solar cells were in the range of about 4% to about 6%. For example, U.S. Pat. No. 3,880,633 (Jordan et al.) discloses a spray coating process for making solar cells including an SnO.sub.x layer, a CdS layer and a Cu.sub.2 S layer on a large scale basis, i.e., as a continuous strip 10 feet wide, from which 4 ft. by 8 ft. panels are cut. The Jordan et al. Patent contemplates solar cell efficiencies of only about 5% with efficiencies of up to 8% thought to be feasible. U.S. Pat. No. 4,224,355 (Lampkin et al.) discloses the formation of large area photovoltaic cells (e.g., four square feet) having SnO.sub.x, CdS (or ZnS or ZnCdS) and Cu.sub.2 S layers using a spray coating process. The Lampkin et al. Patent appears also to contemplate efficiencies of only about 5%. U.S. Pat. Nos. 4,125,391 (Van Laethem) and 4,123,244 (Leclercq et al.) disclose spray processes for forming a coating on a glass substrate. The Van Laethem and Leclercq et al. Patents disclose that the substrate may be in excess of one meter wide. U.S. Pat. Nos. 4,327,119 (Lis et al) and 4,336,285 (Squillante) disclose spray processes for making photovoltaic cells having ZnCdS, CdTe and Aquadag layers on an indium tin oxide coated glass substrate.
It has been difficult, however, to economically provide large area thin films of uniform good quality which are devoid of inhomogenuities and discontinuties by such processes, particularly for large area solar cells. It has also been difficult to increase the efficiency of solar cells made by such processes.